MOS type/World leader in gas sensing innovation

Operating principle

Figaro offers a wide range of gas sensors for the detection of various gases, from explosive gases such as propane, toxic gases such as carbon monoxide, to air quality sensors for volatile organic compounds (VOCs) that are responsible for sick-house syndrome. Figaro offers a diverse portfolio of sensor technologies that can be matched to the unique requirements of each application.

  • MOS type
  • Catalytic type
  • Electrochemical type

Summary

STEP1

In clean air, donor electrons in tin dioxide are attracted toward oxygen which is adsorbed on the surface of the sensing material, preventing electric current flow.

STEP2

In the presence of reducing gases, the surface density of adsorbed oxygen decreases as it reacts with the reducing gases. Electrons are then released into the tin dioxide, allowing current to flow freely through the sensor.

Operating principle

In the most extreme case where oxygen concentration is 0%, when metal oxide sensor material (typically tin dioxide [SnO2-x]) is heated at high temperature such as 400˚C, free electrons flow through the conjoined parts (grain boundary) of tin dioxide crystals. In clean air (approx.. 21% O2), oxygen is adsorbed on the metal oxide surface. With its high electron affinity, adsorbed oxygen attracts free electrons inside the metal oxide, forming a potential barrier (eVs in air) at the grain boundaries. This potential barrier prevents electron flow, causing high sensor resistance in clean air.

When the sensor is exposed to combustible gas or reducing gas (such as carbon monoxide), the oxidation reaction of such gas with adsorbed oxygen occurs at the surface of tin dioxide.

As a result, the density of adsorbed oxygen on the tin dioxide surface decreases, and the height of the potential barrier is reduced. Electrons easily flow through the potential barrier of reduced height, and the sensor resistance decreases. Gas concentration in air can be detected by measuring the resistance change of MOS-type gas sensors. The chemical reaction of gases and adsorbed oxygen on the tin dioxide surface varies depending on the reactivity of sensing materials and working temperature of the sensor.

Warning and precautions for use of MOS-type gas sensors

  • Carefully read product information and other technical information provided by Figaro before using our products, and confirm specifications and operating conditions.
  • When designing an application circuit, please make sure that an accidental short circuit or open circuit of other electronic components would not cause the sensor to be subjected to excessive voltage, current, or temperatures exceeding the rated values.
  • When designing application products, please make sure that a gas sensor malfunction would not 1) cause adverse effects on other components, 2) directly or indirectly impair the safety of application products that use gas sensors (e.g., emit smoke, cause fire, or other unstable states of application products).
  • Consider adding safety measures for fail-safe where necessary, such as a protection circuit.

Cautions for safe use of MOS-type gas sensors

Applied voltage
Do not use the gas sensor if higher than the rated voltage is applied. If higher than the rated voltage is applied to the sensor, the lead wires, the heater, and/or the sensor element may be damaged or sensor characteristics may be irreversibly impaired, even if no physical damage or breakage occurs.
Environmental conditions
  • Avoid exposing the sensor where adhesives or hair grooming materials containing silicone or silicone rubber/putty may be present. If silicone vapors adsorb onto the sensing element surface, the sensing material will be coated, irreversibly inhibiting sensitivity.
  • Avoid highly corrosive environments. High density exposure to corrosive gases such as hydrogen sulfide, sulfur oxide, chlorine, hydrogen chloride, etc. for extended periods may cause corrosion or breakage of the lead wires or of the heater material. For information on specific gases and conditions for corrosive gases, please consult with Figaro.
  • Avoid contamination by alkaline metals. Sensor characteristics may be significantly changed if the sensor is contaminated by alkaline metals, especially salt water spray.
  • Sensor performance may be affected if exposed to a high density of reactive gases for a long period of time, regardless of the powering condition. For information on specific gases and conditions, please consult with Figaro.
  • If water freezes on the sensing element surface, the sensing material may crack, which will irreversibly affect sensor characteristics.
  • If water condenses on the sensor element surface and remains for an extended period, sensor characteristics may temporarily drift. Light condensation under normal conditions of indoor usage would not pose a significant problem for sensor performance.
  • Regardless of its powering condition, if the sensor is exposed in extreme conditions such as very high humidity, high temperatures, or high contamination levels of organic vapors or other gases for a long period of time, sensor performance may be impaired.
  • MOS-type gas sensors cannot properly operate in a zero or low oxygen content atmosphere. They require the presence of normal ambient oxygen in their operating environment in order to function properly.
Handling
  • Sensor characteristics may be changed due to soaking or splashing the sensor with water.
  • Avoid mechanical shock. Breakage of lead wires may occur if the sensor is subjected to a strong shock.
  • Under no circumstances should the sensor be disassembled, nor should the sensor can and/or cap be deformed. Such action would void the sensor warranty and would cause irreversible change in characteristics.
Storage conditions
When stored without powering in normal air for a long period, or in an environment contaminated with organic vapors or volatile oils, the sensor may show a reversible drift in resistance according to the environment. The sensor should be stored in a sealed bag made of material that does not emit odor or gas. Do not store the sensor with silica gel.
Application product design
  • When stored without powering for a long period, the sensor may show a temporary drift in its response depending on the environment in which it was stored. Such drift is reversible. Powering the sensor will offset this drift and stabilize the sensor. The longer the unpowered storage period, the longer the conditioning period required to stabilize the sensor before usage.
Mounting process
  • Manual soldering is always recommended for mounting gas sensors.
  • Wave soldering may be used for MOS-type gas sensors if limited to the following conditions:
    (1) Suggested flux: Rosin flux with minimal chlorine
    (2) Transfer speed: 1-2 meters / min.
    (3) Pre-heating temperature: 100±20˚C
    (4) Solder bath temperature: 250±10˚C
    (5) Allowable soldering passes: 2 times maximum
    The results of wave soldering cannot be guaranteed if conducted outside the above guidelines since some flux vapors may cause poisoning or drift in sensor performance similar to the effects of silicone vapors.
  • When a resin coating is applied on a printed circuit board for improving its resistance to moisture and corrosive gases, the chemical solvent contained in the coating material may affect sensor characteristics. Sample testing should be conducted to see if this process would adversely affect sensor characteristics.
  • Excessive vibration may cause the sensor element and/or the lead wires to resonate and eventually break. Usage of compressed air drivers or ultrasonic welders on assembly lines may cause such vibration to the sensor. Before using such equipment, preliminary tests should be conducted to verify that there will be no influence on sensor characteristics.
  • MOS type
  • Catalytic type
  • Electrochemical type